Field of the Invention
The invention relates to an integrated circuit configuration having a crystal oscillator for producing an operating clock signal, including a crystal to be connected externally at first and second connection terminals for producing a phase shift of 180.degree., and an integrated inverter for producing a further phase shift of 180.degree., thus together effecting an overall phase shift of 360.degree., the inverter having an input and an output connected to the first and the second connection terminals.
Many integrated circuits, such as microcontrollers, in which a multiplicity of peripheral units are additionally integrated besides a microprocessor, have an oscillator which is suitable for the connection of a crystal. The Siemens data book entitled "Microcomputer Components", Microcontrollers, Data Catalog 1998, page 78, lower left-hand figure, shows how such an oscillator can be connected or wired up externally with a crystal. FIG. 2 of the instant application shows an additional internal circuit section which is described in detail below.
U.S. Pat. No. 4,571,5583 describes a crystal oscillator circuit in which the feedback path of the inverter contains two integrators that together effect a phase shift of 180.degree.. That phase shift through 180.degree. causes the oscillatory response of the oscillator to be less temperature-dependent and to be less dependent on manufacturing tolerances. A further generic circuit configuration having a crystal oscillator is described in U.S. Pat. No. 4,321,562.
With rising performance requirements, microcontrollers are being operated at higher and higher frequencies. The crystal disks therefore have to be made thinner and thinner, which means that the risk of breakage increases. That is particularly relevant in the automotive sector due to vibration, since such microcontrollers are used in that sector for engine control, ABS control or the like. Such crystal breakage causes the oscillator to stop and thus also leads to the failure of the entire system. In order to avoid that, a so-called emergency clock signal is often required, which can be achieved by additional circuit complexity.
Published European Patent Application 0 286 879 A1, corresponding to U.S. Pat. No. 4,816,776, discloses an integrated circuit configuration for monitoring an additionally integrated oscillator, in which an integrated oscillator is excited by an external frequency standard. A second integrated reserve oscillator produces a slower auxiliary clock signal which is constantly compared with the oscillator clock signal, and the system clock signal is then obtained from the faster clock signal. If the crystal fails, for example as a result of it breaking, the system changes over to the reserve oscillator.
A further possibility is using a PLL for producing the system clock signal, in which that PLL has a minimum frequency which depends on the minimum frequency of the voltage-controlled oscillator in the PLL. In the event of the crystal breaking, the frequency of the system clock signal reverts to the minimum frequency of the PLL. A further advantage of a PLL is that the oscillator frequency can be many times lower than that of the system clock signal, which means that crystals which are thicker and therefore less likely to break are used.
The disadvantage of all of those configurations is the relatively large amount of circuitry.